Precise Characterization of Individual Microfluidic Droplets Using Laser Diffraction.

The expanding use of microfluidic droplets and particles across disciplines, from biology to materials science, highlights the need for developing precise characterization methods. Conventional particle characterization based on light scattering typically relies on averaged data from multiple particles, which can lead to inaccuracies due to contamination from larger particles. To overcome this issue, we here present a versatile laser diffraction (LD) system for characterizing individual droplets and particles flowing in a poly-(dimethylsiloxane) (PDMS) microfluidic device. Our system, mounted on a commercial inverted microscope, facilitates the simultaneous estimation of both the diameter and the refractive index of microparticles and droplets of size 20-50 μm. The LD system captures the angular distribution of scattered light from individual droplets as they pass through the PDMS microfluidic channels. Validation experiments were performed using liquid paraffin with varying refractive indices, oil-in-water (O/W) and water-in-oil (W/O) droplets, and size-certified polystyrene beads. Results showed high accuracy, with mean diameter estimation errors under 5% and refractive index estimation errors <0.5%. This adaptable characterization system can be combined with various microfluidic systems for droplet and particle generation, mixing, and sorting, offering broad potential for applications in multiple research domains.